[llvm] [llvm][ELF]Add Shdr check for getBuildID (PR #126537)

[James Henderson via llvm-commits]

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@@ -40,22 +93,23 @@ TEST(BuildIDTest, InvalidNoteFileSizeTest) {
   Data:           ELFDATA2LSB
   Type:           ET_EXEC
   Machine:        EM_X86_64
+ProgramHeaders:
+  - Type:         PT_NOTE
+    FirstSec:     .note.gnu.build-id
+    LastSec:      .note.gnu.build-id
 Sections:
   - Name:         .note.gnu.build-id
     Type:         SHT_NOTE
     AddressAlign: 0x04
+    Offset:       0x8000
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jh7370 wrote:

There are two similarly-named fields in yaml2obj, but with different meanings:
1) `Offset`: this says "put the section at the specified offset in the file (which will increase the file size to fit)" and "set the sh_offset field of the section header to the specified value".
2) `SHOffset`: "put the section at the offset specified by `Offset` (if present) or at the next possible valid offset (if not present), but set the section header sh_offset field to the specified value.

Using `Offset` like you've done means there's no invalid section header, since the object file will just be increased to fit the section in at the specified location. Using `SHOffset` will place the section data in a valid place (and the program header will point to it), but the section header itself will be incorrect. Since the details of the section are read from the section header in the first place, tools will try to read the section at the invalid `SHOffset` value, which will be outside the file and generate an error, causing the code to fall back to reading via the program headers.

For an example, see https://github.com/llvm/llvm-project/blob/15c49b9db3f60bdbd320271d5e97f118c00b95dd/llvm/test/tools/yaml2obj/ELF/override-shoffset.yaml#L22.

https://github.com/llvm/llvm-project/pull/126537